[TowerTalk] R: Roof mounted mast, Antenna performance

[Maurizio Panicara]

To avoid elements detuning and coupling losses, and to minimize straight up
radiation, an half wavelenght from a roof that's comparable or larger than
the antenna size is the good choice.
A quarter wavelenght (and even less if the roof is made of an isolating
material) could be a sufficient height from roof but the overall performance
depends if the building itself is tall enough to set the antenna at least
half wavelenght from the real ground or over the average buildings height.
Again, the wave reflection relative to a low elevation angle occur several
wavelenghts from the antenna and, in case of built up areas, the zenital
pattern predictions are very complicated and different at different
headings.
Just to point it out, not always nearby buildings are a damage on low angles
and sometimes antennas at low height (would have no low angles over flat
unobstructed areas) benefit the screening effect of phase reversed waves
(those canceling radiation at low angles) and perform similarly on low
angles (-3 dB) to antennas on much taller towers.


73,
Mauri I4JMY


----- Original Message -----
From: John Buck <kh7t@arrl.net>
To: Maurizio Panicara <i4jmy@iol.it>; <towertalk@contesting.com>
Cc: Al Williams <alwilliams@olywa.net>
Sent: Monday, August 07, 2000 10:59 PM
Subject: Roof mounted mast, Antenna performance


> How high is high enough above conventional house sheet metal roof areas
> ranging from 1800 to 3000 square feet?  This is a difficult question
> given the wide variations in roof bonding condition, pitch and shape.
> But what are realistic tradeoffs to get good performance for a 2 or 3
> element yagis or Quads on short roof mounted masts on one or two story
> houses?
>
> John KH7T
>



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